MRI is an effective, non-invasive magnetic imaging technique for generating sharp images of the internal anatomy of the human body, which provides an efficient means for diagnosing disorders such as neurological and cardiac abnormalities and for spotting tumors and the like. Briefly, the patient is placed within the center of a large superconducting magnetic that generates a powerful static magnetic field. The static magnetic field causes protons within tissues of the body to align with an axis of the static field. A pulsed radio-frequency (RF) magnetic field is then applied causing the protons to begin to precess around the axis of the static field. Pulsed gradient magnetic fields are then applied to cause the protons within selected locations of the body to emit RF signals, which are detected by sensors of the MRI system. Based on the RF signals emitted by the protons, the MRI system then generates a precise image of the selected locations of the body, typically image slices of organs of interest.
However, MRI procedures are problematic for patients with implantable medical devices such as pacemakers and ICDs. A significant problem is that the strong fields of the MRI can induce currents within the lead system that cause the electrodes of leads of the implantable device to become significantly heated, potentially damaging adjacent tissues or the lead itself. Heating is principally due to the RF components of the MRI fields. In worst-case scenarios, the temperature at the tip of an implanted lead can increase as much as 70 degrees Celsius (C.) during an MRI. Although such a dramatic increase is probably unlikely within a system wherein leads are properly implanted, even a temperature increase of only about 8°-13° C. can cause myocardial tissue damage. Furthermore, any significant heating of the electrodes of pacemaker and ICD leads, particular tip electrodes, can affect pacing and sensing parameters associated with the tissue near the electrode, thus potentially preventing pacing pulses from being properly captured within the heart of the patient and/or preventing intrinsic electrical events from being properly sensed by the device. The latter may potentially result, depending upon the circumstances, in therapy being improperly delivered or improperly withheld. Another significant concern is that any currents induced in the lead system can potentially generate voltages within cardiac tissue comparable in amplitude and duration to stimulation pulses and hence might trigger unwanted contractions of heart tissue. The rate of such contractions can be extremely high, posing significant clinical risks on patients.
Hence, there is a need to reduce heating in the leads of implantable medical devices, especially pacemakers and ICDs, and to also reduce the risks of improper tissue stimulation during an MRI, which is referred to herein as MRI-induced pacing.
Various techniques have been developed to address these problems. See, for example, the following patents and patent applications: U.S. Pat. Nos. 6,871,091; 6,930,242; 6,944,489; 6,971,391 6,985,775; 7,050,855; 7,164,950; 7,489,495; U.S. Patent Application Nos. 2003/0144718, now abandoned, and 2003/0144719, now abandoned; as well as the following PCT documents WO03/037424, WO03/063946, WO03/063953. At least some of these techniques are directed to detecting MRI fields and to electrically disconnecting electrodes from the implantable device in an effort to prevent current loops from being generated that might induce lead heating, particularly tip heating. See, also, U.S. Pat. No. 7,369,898 to Kroll et al., entitled “System and Method for Responding to Pulsed Gradient Magnetic Fields using an Implantable Medical Device.”
Various aspects of the invention are directed to providing MRI-based lead switching or band stop filtering systems and methods.